There are many known methods of mercury analysis, including: colorimetric; atomic absorption coupled to vapour generation; atomic absorption following gold trapping from vapour generation; and atomic fluorescence coupled to vapour generator.
Atomic fluorescence is commonly achieved in a Cold Vapour Atomic Fluorescence Spectrophotometer (CVAFS). CVAFS is preferred, as compared to atomic absorption, since the phenomena is linear over a much wider range and is not subject to positive interferences. Rather, it can be subject to negative interference modes, with certain molecular species causing quenching. The present invention provides techniques for improving the performance of such a device and for overcoming negative interference.
It is known that gold is an excellent adsorber of mercury, which forms a gold amalgam with mercury. However, many conventional instruments suffer from a "memory" effect. This arises due to the use of gold having substantial thickness in a detector cell, and mercury migrating from the surface of the gold to the interior, or at least below the surface. Consequently, when the mercury is flushed from a detector cell by heating in an inert gas flow, in known manner, not all the mercury is immediately released. In subsequent cycles, mercury from below the gold surface can migrate to the surface and be flushed out, giving a false mercury reading. While it has been proposed to use such materials as gold-coated sand, with the intention of providing a gold film so thin as to prevent this problem, such technique has its own drawbacks. The gold-coated sand is not always completely stable, and the gold may not remain plated or adhered to the sand particles, particularly when subjected to hundreds and thousands of heating and cooling cycles.
Pure gold is very selective and does not adsorb most contaminants that can give false readings, which may contaminate the flow path of the instrument. However, problems have often been reported with the adsorption of competing compounds. Thus, the activity of the gold can be taken up by other compounds, and once used, the gold in the cartridge will not be able to capture mercury. This will give a false, low reading. During desorption, the competing compounds may be released. They may then register a false positive reading, attenuate the actual mercury signal, or contaminate the flow path. Organic compounds and water vapour are common examples of contaminants that can be unintentionally entrained.
In conventional instruments, the flows of different gases, such as air, carrier gas, are not controlled so as to give optimum performance. No consideration has been given to optimizing the flows of such gases.
In many conventional instruments, ambient air and other contaminants can be passed through the detector cell. To maintain the purity of the detector cell, and prevent contamination, it is desirable that only a carrier gas, containing mercury when present, be passed through the detector cell.
Common types of detector cells have the interior optical path filled with air. The ultraviolet radiation produced by the lamp of the detector produce compounds in air which absorb the ultraviolet light. The most important reaction is the break down of oxygen and the creation of free radicals that recombine to produce ozone. These UV adsorbent compounds decrease sensitivity of the detector and cause significant baseline shift.
To calibrate known instruments, it is necessary to provide manual injections using gas tight syringes. This is cumbersome and awkward, and necessarily prevents any automation of the device.
Accordingly, it is desirable to provide a mercury detector, based on Cold Vapour Atomic Fluorescence Spectrophotometry which provides a much higher degree of sensitivity. It is desirable that such a detector not suffer from any memory effects of the gold, and not be susceptible to contaminants entering the detector cartridges. It is desirable that the gas flows be controlled to optimise, or at least improve, usage of the gases and performance of the apparatus as a whole.
It is further desirable that the apparatus be capable of automatic operation, and include means for automatic recalibration.
It is also desirable that the detector itself not be susceptible to the generation of ultraviolet absorbent compounds which would decrease sensitivity.